Magnetic speed sensor

ABSTRACT

A speed sensor for a vehicle wheel has a rotating tone wheel with a stationary magnetic sensor having a pair of sensing poles positioned proximate the tone wheel teeth. Coil means are wound on the poles and the poles are magnetized to induce a signal voltage in the coil means corresponding to wheel speed. The poles are phased relative to the teeth on the tone wheel to reduce noise voltage components in the wheel speed signal voltage due to axial misalignment and vibration of the tone wheel. The poles may be phased 180° relative to one another with respect to the teeth on the tone wheel so that one pole is in registry with a tooth when the other pole is in registry with a space between two teeth. With this arrangement the combined reluctance of the poles is approximately constant.

The invention relates to adaptive braking systems for use on vehiclessuch as automobiles or trucks and more specifically to wheel speedsensors for providing the adaptive braking system with informationrelating to the speed and acceleration of the wheels. The invention canbe used in the adaptive braking system described in U.S. Pat. No.3,494,671 and may be applied to the wheel speed sensors described inU.S. Pat. Nos. 3,626,225; 3,626,226; 3,626,227; 3,626,228 and 3,629,635.All of the above patents are assigned to the same assignee as thepresent application.

The present invention reduces the effects of axial tone wheelmisalignment and axial tone wheel vibration. Axial tone wheelmisalignment, such as run out or wobble, causes low frequency noisecomponents in the speed signal. Axial tone wheel vibration causes highfrequency noise components in the speed signal. In adaptive brakingsystems the speed signal is usually time differentiated to obtain anacceleration signal and this causes the noise components to reachexcessive values. Of course, the low frequency noise components can bereduced to an acceptable level by close manufacturing tolerances andaccurate alignment of the tone wheel, but the cost of this solution ismuch too high for automotive applications. The speed signal must befiltered to reduce the high frequency noise components.

The invention contemplates a speed sensor for a vehicle wheel comprisinga tone wheel driven by the vehicle and subject to axial misalignment andvibration, the tone wheel being made of magnetic material and having aplurality of teeth, and sensing means having a pair of poles of magneticmaterial proximate the tone wheel teeth, coil means on the poles, andmeans for magnetizing the poles to induce a signal voltage in the coilmeans corresponding to the wheel speed upon rotation of the tone wheelrelative to the sensing means, the poles on the sensing means beingphased relative to the teeth on the tone wheel to reduce noise voltagecomponents in the wheel speed signal voltage due to axial misalignmentand vibration of the tone wheel.

The present invention reduces the noise components to an acceptablelevel even when the tone wheel is warped and axially misaligned andsubject to severe vibrations by using two sensing poles located radiallyproximate the tone wheel teeth and separated from one anothercircumferentially of the tone wheel to provide a suitable phasedifference between the flux in the two poles. In the preferredembodiment the phase difference is 180° so that one pole is adjacent atooth when the other pole is adjacent a space between two teeth. Asingle sensing coil may be wound on both poles or a sensing coil may bewound on each pole with the sensing coils connected in series oppositionand preferably having the same number of turns.

The wheel speed signal components A sin wt add and the lower frequencynoise components due to axial misalignment cancel. The low frequencynoise components V₀ may be considered constant for a few cycles of thesignal component so that the voltages V₁ and V₂ induced in the separatesensing coils are:

    V.sub.1 =V.sub.0 +A sin wt                                 (1)

    V.sub.2 =V.sub.0 -A sin wt                                 (2)

    V.sub.1 -V.sub.2 =2A sin wt                                (3)

With the sensing coils connected in series opposition the low frequencynoise components cancel and the signal voltages induced in the sensingcoils are added.

The high frequency noise components due to vibration are reducedconsiderably unless the high frequency noise components are exactly inphase with the signal voltage. In one position of the tone wheelrelative to the sensor poles, the reluctance between the tone wheel andboth sensors is the same and the vibration noise components arecancelled almost completely in this position since the same voltageamplitude is generated in each sensing coil. When one sensing pole isadjacent a tooth and the other sensing pole is adjacent a space betweentwo teeth the difference in reluctance between the tone wheel andsensors is maximum and the difference in signals including the highfrequency components in the two sensing coils is maximum.

With a single coil wound on both sensing poles, as the tone wheelrotates the flux through the coil changes direction as the polesalternately are positioned adjacent the teeth and the change of fluxthrough the coil generates a speed signal. Axial vibration causing highfrequency noise components increases the flux through both poles and,because the flux is in opposite directions, it effectively cancels. Thedegree of cancellation of the high frequency noise components depends onthe tone wheel position relative to the sensing poles and the greatestcancellation occurs when both poles have equal flux paths. Low frequencynoise components caused by wheel misalignment increases the flux in bothpoles as the tone wheel moves towards the poles and the noise componentscancel since the flux through the coil is in opposite directions.

In another arrangement, the two sensors are phased 90° apart and thisconfiguration may be more effective in cancelling the high frequencycomponent due to tone wheel vibration than the arrangements describedbriefly above since the maximum difference in reluctance of the twopoles is less. However, this arrangement may not be as effective incancelling the low frequency components due to tone wheel misalignmentas the arrangements described briefly above.

With a phase difference of 180° between the poles, that is, when onepole is adjacent a tooth and the other pole is adjacent a space betweentwo teeth, a constant reluctance load is presented to the permanentmagnet and this reduces noise resulting from tone wheel misalignment andvibration. As the tone wheel rotates the permeance of each pole variesapproximately sinusodially and the phase difference is 180°. Thecombined permeance of the two poles is approximately constant as shownby the following equations:

    P.sub.1 =P.sub.0 +ΔP sin θ                     (4)

    P.sub.2 =P.sub.0 -ΔP sin θ                     (5)

    P.sub.1 +P.sub.2 =2P.sub.0                                 ( 6)

Where

P is steady state permeance due to air gap,

P₁ is the permeance of pole 1,

P₂ is the permeance of pole 2,

ΔP sin θ is the permeance variation due to tone wheel rotation.

The reluctance R also is constant since it is the reciprocal ofpermeance and is given by the following equation: ##EQU1##

When the reluctance of the wheel speed sensor varies a minor magnetichysteresis loop is generated each time a tooth passes a sensor pole andthe sensor inductance varies with the position of a tooth relative to apole. This distorts the speed signal waveform resulting in speed signalfrequency modulation noise. With constant reluctance the minor magnetichysteresis loop does not occur and there is no variation in sensorinductance and the speed signal is a sinusoidial waveform withoutappreciable distortion.

BRIF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a wheel speed sensor constructedaccording to the invention,

FIG. 2 is a wiring diagram showing the coils of FIG. 1 connected inseries opposition,

FIG. 3 is a schematic diagram partly in section of a second embodimentof a wheel speed sensor constructed according to the invention,

FIG. 4 is an end view of the embodiment shown in FIG. 3,

FIG. 5 is a front view of the embodiment shown in FIG. 3,

FIG. 6 is a schematic diagram partly in section of a third embodiment ofa wheel speed sensor constructed according to the invention,

FIG. 7 is an end view of the embodiment shown in FIG. 6,

FIG. 8 is a front view of the embodiment shown in FIG. 6,

FIG. 9 is an isometric view of a fourth embodiment of a wheel speedsensor constructed according to the invention, and

FIG. 10 is a schematic diagram of a fifth embodiment of a wheel speedsensor constructed according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 of the drawings, the wheel speed sensor showntherein and constructed according to the invention includes a tone wheel1 of magnetic material having a series of equally spaced teeth 3 aboutits periphery. The tone wheel may be secured to the vehicle wheel huband may be of relatively large diameter or it may be frictionally drivenand of relatively small diameter as described in the above patents. Thefriction drive may be operated by the wheel or drive shaft or in anyother suitable manner.

A magnetic sensor 5 has a pair of sensing poles 7 and 9 secured to theends of a permanent magnet 11 in U-shaped configuration. Coils 13 and 15are wound on sensing poles 7 and 9, respectively, and the coils areconnected in series opposition as shown in FIG. 2.

The magnetic sensor is mounted on a stationary part of the vehicle withpoles 7 and 9 located radially at tooth centers proximate the tonewheel. The poles are spaced relative to teeth 3 so that one pole isadjacent a tooth when the other pole is adjacent a space between twoteeth. This arrangement provides 180° phase difference between the fluxin the two poles and presents a constant reluctance load to thepermanent magnet.

In FIGS. 3, 4 and 5 the magnetic sensor 25 comprises a pair of sensingpoles 27 and 29 secured to a permanent magnet 31. A single coil 33 iswound on both sensing poles. The poles are spaced relative to teeth 3 onthe tone wheel so that one pole is adjacent a tooth when the other poleis adjacent a space between two teeth to provide a 180° phase differencebetween the flux in the two poles and to present a constant reluctanceload to the permanent magnet. The poles are shaped so that the endsadjacent the teeth are substantially smaller than the remainder of thepoles and preferably smaller than the depth of the teeth on the tonewheel.

In FIGS. 6, 7 and 8, the magnetic sensor comprises a U-shaped pole piece45 having poles 47 and 49. The pole piece is secured at the base of theU to a permanent magnet 51. Sensing coils 53 and 55 are wound on poles47 and 49, respectively, and are connected in series opposition as shownin FIG. 2. A U-shaped shield 57 is secured to permanent magnet 31 andprovides a return path for magnetic flux and shields the sensor fromelectromagnetic noise. Poles 47 and 49 are located radially at toothcenters proximate the tone wheel and one arm of U-shaped shield 57 ispositioned proximate the tone wheel spaced from the teeth. The poles arespaced relative to teeth 3 so that one pole is adjacent a tooth when theother pole is adjacent a space between two teeth to provide a 180° phasedifference between the flux in the two poles and to present a constantreluctance load to the permanent magnet.

In FIG. 9 the magnetic sensor comprises a U-shaped pole piece 75 havingpoles 77 and 79. One end of a permanent magnet 81 is secured to polepiece 75. Sensing coils 83 and 85 are wound on poles 77 and 79,respectively, and are connected in series opposition as shown in FIG. 2.A third pole 87 is secured to the other end of permanent magnet 81 anddirects the return flux from the tone wheel to the magnet similarly tothe embodiment of FIGS. 6, 7 and 8. Poles 77 and 79 are located radiallyat tooth centers proximate the tone wheel and pole 87 is positionedproximate the tone wheel spaced from the teeth. Poles 77 and 79 arespaced relative to teeth 3 so that one pole is adjacent a tooth when theother pole is adjacent a space between two teeth to provide 180° phasedifference between the flux in the two poles and to present a constantreluctance load to the permanent magnet.

The embodiment shown in FIG. 10 is substantially the same as theembodiment shown in FIG. 1 except that sensing poles 97 and 99 arespaced relative to teeth 3 so that one pole is adjacent a tooth when onehalf of the other pole is adjacent a space between two teeth and theother half of the pole is adjacent a tooth. This arrangement provides a90° phase difference between the flux in the two poles.

Although the reluctance in this arrangement is not constant it is moreeffective in cancelling vibration noise than the other arragements wherethe poles are phased 180° because the maximum difference in reluctancebetween the two poles is less than in those arrangements.

As explained above, this arrangement is effective in cancelling highfrequency components due to tone wheel vibration because the maximumdifference in reluctance of the two poles is small.

In all the above embodiments of the invention noise components arereduced to an acceptable level even when the tone wheel is warped andaxially misaligned and subject to severe vibrations. Low frequency noisecomponents due to axial misalignment cancel and high frequency noisecomponents due to axial vibration are reduced considerably. A constantreluctance load is presented to the permanent magnet to prevent minormagnetic hysteresis loops from occuring and there is no variation insensor inductance. The speed signal is a sinusodial waveform withoutappreciable distortion.

What is claimed is:
 1. A speed sensor for a vehicle wheel comprising atone wheel driven by the vehicle and subject to axial misalignment andvibration, the tone wheel being made of magnetic material and having aplurality of teeth, and sensing means having a pair of poles of magneticmaterial proximate the tone wheel teeth, coil means on the poles, and apermanent magnet having one of its poles connected to the poles on thesensing means for magnetizing the poles on the sensing means to induce asignal voltage in the coil means corresponding to wheel speed uponrotation of the tone wheel relative to the sensing means, the poles onthe sensing means being phased relative to the teeth on the tone wheelto reduce noise voltage components in the wheel speed signal voltage dueto axial misalignment and vibration of the tone wheel, and asubstantially U-shaped shield connected to the other pole of thepermanent magnet and enclosing a substantial portion of the sensingmeans from electromagnetic noise, a portion of the U-shaped shield beingpositioned adjacent the tone wheel remote from the teeth to provide areturn path for magnetic flux.
 2. A speed sensor as described in claim 1in which the poles on the sensing means are spaced relative to the tonewheel teeth so that one of the poles is in registry with a tooth whenthe other pole is in registry with a space between two teeth.
 3. A speedsensor as described in claim 1 in which the poles on the sensing meansare phased 180° relative to one another with respect to the teeth on thetone wheel.
 4. A speed sensor as described in claim 1 in which the coilmeans includes a separate coil wound on each sensing pole and the coilsare connected in series opposition.
 5. A speed sensor as described inclaim 1 in which the poles on the sensing means are phased relative tothe teeth on the tone wheel so the combined reluctance of the poles isapproximately constant.
 6. A speed sensor for a vehicle wheel comprisinga tone wheel driven by the vehicle and subject to axial misalignment andvibration, the tone wheel being made of magnetic material and having aplurality of teeth, and sensing means having a pair of poles of magneticmaterial proximate the tone wheel teeth, coil means on the poles, and apermanent magnet having one of its poles connected to both poles of thesensing means for magnetizing the poles to induce a signal voltage inthe coil means corresponding to wheel speed upon rotation of the tonewheel relative to the sensing means, the poles on the sensing meansbeing phased relative to the teeth on the tone wheel to reduce noisevoltage components in the wheel speed signal voltage due to axialmisalignment and vibration of the tone wheel, and the sensing meansincluding a third pole connected to the other pole of the permanentmagnet and positioned proximate the tone wheel remote from the teeth toprovide a return path for magnetic flux.
 7. A speed sensor as describedin claim 6 in which the pair of poles on the sensing means are spacedrelative to the tone wheel teeth so that one of the poles is in registrywith a tooth when the other pole is in registry with a space between twoteeth.
 8. A speed sensor as described in claim 6 in which the pair ofpoles on the sensing means are phased 180° relative to one another withrespect to the teeth on the tone wheel.
 9. A speed sensor as describedin claim 6 in which the coil means includes a separate coil wound oneach sensing pole of the pair and the coils are connected in seriesopposition.
 10. A speed sensor as described in claim 6 in which the pairof poles on the sensing means are phased relative to the teeth on thetone wheel so the combined reluctance of the poles is approximatelyconstant. .Iadd.
 11. A speed sensor as claimed in claim 1, in which thecoil means includes a separate coil wound on each pole piece and thecoils are connected in series opposition. .Iaddend..Iadd.
 12. A speedsensor for a vehicle wheel comprising a tone wheel driven by thevehicle, the tone wheel being made of magnetic material and having aplurality of equally-spaced teeth defining spaces therebetween, andsensing means mounted adjacent said tone wheel, said sensing meanscomprising a pair of pole pieces located adjacent said teeth, and apermanent magnet located between said pole pieces for providing a fluxpath therebetween, said magnet having a pair of opposite ends havingopposite magnetic polarities, one of the opposite ends of said magnetbeing magnetically connected to one of the pole pieces, the other of theopposite ends of the magnet being magnetically connected to the other ofsaid pole pieces, and coil means associated with the pole pieces forproviding an output signal in response to changes in the magnetic fluxin said pole pieces. .Iaddend..Iadd.
 13. A speed sensor as claimed inclaim 12, in which the axis of the permanent magnet is parallel to theaxis of rotation of the tone wheel. .Iaddend..Iadd.
 14. A speed sensoras claimed in claim 12, in which the axis of the permanent magnet isparallel to the plane in which the tone wheel rotates. .Iaddend..Iadd.15. A speed sensor for a vehicle wheel comprising a tone wheel driven bythe vehicle, the tone wheel being made of magnetic material and having aplurality of equally-spaced teeth defining spaces therebetween, andsensing means mounted adjacent said tone wheel, said sensing meanscomprising a pair of pole pieces located adjacent said teeth, and apermanent magnet defining an axis and a pair of ends of oppositemagnetic polarity, said magnet being located between said pole piecesand disposed so that the axis of the magnet extends parallel to theplane in which said tone wheel rotates, one end of the magnet beingmagnetically connected to one of the pole pieces so that said one polepiece is magnetized at the polarity of said one end of the magnet, theother end of the magnet being magnetically connected to the other polepiece so that the other pole piece is magnetized at the polarity of theother end of the magnet and opposite to the polarity of the one polepiece, and coil means associated with the pole pieces for providing anoutput signal in response to changes in the magnetic flux in said polepieces. .Iaddend..Iadd.
 16. A speed sensor as claimed in claim 15, inwhich the pole pieces are spaced relative to the teeth of the tone wheelso that the phase difference between the flux in said pole pieces is180°. .Iaddend..Iadd.
 17. A speed sensor as claimed in claim 15, inwhich the pole pieces are spaced relative to the tone wheel so that oneof the pole pieces is in registry with a tooth when the other pole pieceis in registry with a space between two teeth. .Iaddend..Iadd.
 18. Aspeed sensor as claimed in claim 17, in which the coil means includes aseparate coil wound on each pole piece and the coils are connected inseries opposition. .Iaddend. .Iadd.
 19. A speed sensor for a vehiclewheel comprising a tone wheel driven by the vehicle, the tone wheelbeing made of magnetic material and having a plurality of equally-spacedteeth defining spaces therebetween, and sensing means mounted adjacentsaid tone wheel, said sensing means comprising a pair of pole pieceslocated adjacent said teeth, said pole pieces being spaced so that theaxis of one of said pole pieces is always in registry with one of saidspaces when the axis of the other pole piece is in registry with one ofsaid teeth, and the axis of said other pole piece is always in registrywith one of said spaces when the axis of said one pole piece is inregistry with one of said teeth, regardless of the angular position ofthe tone wheel, so that the phase difference between the flux in saidpole pieces is 180° , means magnetically connecting said pole pieces todefine a flux path between the pole pieces, said last-mentioned meansincluding means for magnetizing the pole pieces so that the pole pieceshave opposite polarity, and coil means associated with the pole piecesfor providing an output signal in response to changes in magnetic fluxin said pole pieces. .Iaddend.